This page describes some reasons why email encryption is good and should be used by everyone. It is not about how encryption works technically.
What is encryption?
Encryption is a mechanism that ensures that only the correct recipient of a message is able to read it. When I send a message to Thomas, it is encrypted, that means it is changed in a way that nobody can read it. Only Thomas is able to change it back to the original message (decrypt it), and will then be able to read it.